A communication technique called OFDM (Orthogonal Frequency Division Multiplexing) is used in wireless transmission and the like. Such technique is a digital modulation technique where a plurality of subcarriers are aligned at predetermined frequency intervals and information is transmitted on the respective subcarriers. Using this technique, it is possible to closely space a plurality of carrier waves without interference between the respective waves and thereby achieve broadband transmission that efficiently uses a narrow range of frequencies. This makes it possible to improve the usage efficiency of frequencies and is suitable for achieving high-speed data transmission.
For example, IEEE (Institute of Electrical and Electronic Engineers) 802.11a that is a standard for wireless LAN (Local Area Networks) carries out wireless transmission using an OFDM technique. Also, according to UWB (Ultra Wide Band) technology, practical use of which has been considered in recent years, a comparatively wide band of high frequencies in a GHz band or the like is used to transmit a multicarrier signal.
FIG. 1 is a diagram showing an example configuration of an existing transmission apparatus that transmits a multicarrier signal. According to the configuration shown in FIG. 1, transmission data obtained at an input terminal 1 is encoded by an encoder 2 and the encoded transmission data is modulated for wireless transmission by a modulator 3. In the transmission data, modulated by the modulator 3, transmission symbols are subjected to an inverse Fourier transform by an Inverse Fast Fourier Transform (IFFT) circuit 4. The transmission symbols subjected to the inverse Fourier transform are sent to a guard interval inserting unit 5, guard intervals are inserted at constant intervals, and the output with the inserted guard intervals is supplied to a digital/analog converter 6 and converted to an analog signal. The converted analog signal is supplied to a wireless transmission unit 7 and is wirelessly transmitted on a predetermined transmission frequency from a connected antenna 8.
However, since the OFDM communication technique uses a carrier with a comparatively wide bandwidth, there is a risk of interference with another carrier being used on another communication network. For example, such a case that a mobile phone unit carries out communication using a predetermined frequency included within the frequency band of the multicarrier signal may be one of the above-described cases. When this type of interference occurs, there are risks that communication quality deteriorates and the communication may be impossible. Therefore, it is necessary to take measures to avoid such interference. One example of a measure that has been investigated in recent years is a technique called DAA (Detect And Avoid). When a multicarrier signal is transmitted, DAA stops transmission on a specific frequency band (for example, in subcarrier units) in the comparatively wide frequency band used for wireless transmission.
FIG. 2 is a diagram showing an example configuration of a transmitter capable of applying the DAA technique. In the example in FIG. 2, the output of the modulator 3 is supplied in subcarrier units to a transmission avoidance circuit 9, information on locations (positions) of subcarriers on which transmission is avoided is obtained from a terminal 9a, and transmission data assigned to the subcarriers designated by such information is replaced with a null signal. A series of transmission symbols partially replaced with the null signal is supplied to the IFFT circuit 4 where processing to orthogonally convert the series to a multicarrier signal is carried out. The remaining processing configuration is the same as the transmission apparatus in FIG. 1. As shown in FIG. 2, by obtaining a multicarrier signal in which the subcarriers with the frequency positions, where transmission is avoided, have been omitted, it is possible to wirelessly transmit a multicarrier signal where transmission is avoided on frequency bands where transmission is to be stopped.
Japanese Unexamined Patent Application Publication No. 2005-27259 issued by the Japanese Patent Office discloses an example of wireless transmission processing for a multicarrier signal.
However, as shown in the example configuration in FIG. 2, when a simplified configuration is used where transmission symbols are removed before carrying out an inverse Fourier transform by the IFFT circuit, a transmission level of the frequency positions (that is, the subcarriers) where transmission is to be stopped will not be sufficiently reduced and it will not be possible to sufficiently prevent interference. For example, in a transmitter configured as shown in FIG. 2, when some of the transmission symbols have been replaced with a null signal, the signal level of the subcarriers assigned the null signal will only be around 15 dB lower than the other subcarriers, and therefore it will not be possible to sufficiently suppress interference from occurring.
As another example of processing that omits specific subcarriers from a multicarrier signal, it would be possible to omit subcarriers by supplying a signal subjected to an inverse Fourier transform by the IFFT circuit to an FIR (Finite Impulse Response) filter and carrying out filtering processing using the FIR filter to omit the subcarriers. However, if an FIR filter is provided, there is a corresponding increase in the circuit scale of the transmission system. In addition, a technology called AIC (Active Interference Cancellation) has been proposed, but in the case where such technology is applied, a complex circuit configuration may be required.
In addition, to properly receive a multicarrier signal where specific subcarriers have been omitted, it is essential that a receiver side accurately figures out that the specific subcarriers were omitted from the signal received. However, with methods proposed in the past, sufficient measures have not been implemented to achieve this and the receiver side has not been able to adequately cope therewith.
The present invention is conceived in view of the above and aims to suitably carry out transmission processing and reception processing when transmitting a multicarrier signal where specific subcarriers are excluded.